The present invention relates generally to the preparation of field effect transistors, and more particularly to the preparation of perovskite conductive oxide electrodes for use in such devices.
Recently, oxide channel field effect transistors (OxFET) have been developed which incorporate perovskite oxides into their design. These devices are similar in architecture to conventional field-effect transistors (FET) with source, drain and gate electrodes, and a channel. However, instead of the use of a semiconductor material, such as silicon, the channel is made from a material capable of undergoing a field induced Mott metal-insulator transition at room temperature. Exemplary channel materials include oxides from the cuprate family of perovskite structure materials related to high temperature superconductors.
Perovskite oxides exhibit a wide range of behavior depending on chemical composition, temperature, electric fields, and magnetic fields. Insulating, metallic and superconducting phases have been identified, and devices based on epitaxial heterostructures such us high-TC Josephson tunnel junctions, superlattices, and the aforementioned oxide channel field effect transistors (OxFET) have been fabricated and are becoming increasingly attractive. Oxide-channel field effect transistors (OxFET) are of particular interest because of the potential to scale such devices beyond the silicon scaling limits, due to the absence of impurity doping in the oxides and because the charge separation layer at the source and drain contacts can be about 1 Angstrom rather than 100 Angstroms.
As stated above, the Mott transition channel in OxFETs is preferably selected to be a perovskite-structure cuprate compound. These materials have the advantage of being compatible with single-crystal materials such as strontium titanate (chemical formula SrTiO3) (hereinafter, STO), lanthanum aluminate, LaAlO3 (hereinafter, LAO), strontium lanthanum aluminate, SrLaAlO4 (hereinafter, SLAO), barium strontium titanate, BaxSr(1xe2x88x92x)TiO3 (hereinafter, BST) or neodymium gallinate, NdGaO4 (hereinafter NGO), all of which are good electrical insulators, making them useful as substrate materials. Furthermore, the perovskite channel materials can be grown epitaxially on single crystal substrates of STO, LAO, SLAO, BST, or NGO. Examples of such channel oxides include, but are not limited to, lanthanum cuprate, La2CuO4 (hereinafter, LCO), yttrium praseodymium barium cuprate, YxPr1xe2x88x92xBa2CU3O7xe2x88x92xcex4 (hereinafter, YPBCO), and yttrium barium cuprate, YBa2Cu3O7xe2x88x92xcex4 (hereinafter YBCO), which are p-type materials; and neodymium cuprate, Nd2CuO4 (NCO) and other n-type materials.
In one OxFET device design, the channel oxide is buried beneath the gate oxide material. FIG. 1 shows a cross section of one such OxFET device 10 in which Mott transition oxide channel 195 is buried under gate oxide material 200. In order to make electrical contact, this design requires buried electrodes, shown as source 60 and drain 70. During fabrication of the device, source 60 and drain 70 electrodes are present on substrate 120 during deposition of the active oxide layers 195 and 200. Because the latter deposition is performed with the substrate at a temperature (T) greater than 500xc2x0 C., and because of their physical proximity to the electrodes, the reactivity and epitaxial compatibility of gate oxide 200 and channel oxide 195 with the electrodes is an important parameter that affects the performance of these complex devices. Source 60 and drain 70 electrodes have traditionally been made from platinum and other conductive metals, but unfortunately, perovskite oxides exhibit contact resistance with such metal electrodes, thereby creating difficulties in fabricating devices with certain degrees of complexity.
Thin film capacitors having epitaxial metal oxide electrodes, such as lanthanum strontium cobalt oxide, LaSrCoO3 (hereinafter LSCO) or lanthanum nickel oxide, LaNiO3 (hereinafter, LNO), have been prepared. Generally, these electrodes are deposited onto single crystal substrates, such as strontium titanium oxide, SrTiO3 (hereinafter STO) or lanthanum aluminum oxide, LaAlO3 (hereinafter, LAO) using pulsed laser ablation techniques. Such work is reported by T. Yu et al. in Mater. Lett. 26, 291-94 (1996). Because these electrodes do not react with perovskite oxides, they provide good electrical contacts for use in capacitors. However, it is impractical to use these electrodes by themselves in buried oxide FET device designs because necessary mechanical and chemical processing techniques either damage the underlying substrate or do not provide an adequate etch stop, thereby precluding good epitaxial growth of an oxide channel.
To get the maximum benefit from buried perovskite oxide FET devices, it is therefore clear that a need still exists for the development of electrodes capable of providing good ohmic (electrical) contact to channel oxides. To minimize contact resistance, such electrodes should be made from materials such as conductive metallic oxides, rather than pure conductive metals. Furthermore, the fabrication of such electrodes should preserve the relevant parts of the substrate for good epitaxial growth of channel oxides. A need also exists for an OxFET structure that incorporates such conductive metal oxide electrodes therein as buried source and drain electrodes and that incorporates the buried oxide channel design. To fulfill these needs, a method of preparing the electrodes and the FET structure is also desirable. The present invention meets these needs.
Briefly, in one aspect, the present invention provides a method for fabricating perovskite oxide electrodes. Such electrodes are particularly useful as buried source and drain electrodes in an OxFET type device having a buried oxide channel. An unexpected advantage of the method for preparing the electrodes is that the underlying substrate is not damaged during the fabrication process. The inventive method for fabricating two electrodes (a first and second electrode), particularly useful as source and drain electrodes, comprises:
(A) providing a substrate;
(B) depositing a layer of a lower conductive oxide onto the substrate;
(C) depositing a coating of an upper conductive oxide onto the layer of the lower conductive oxide;
(D) patterning the upper conductive oxide coating to create a cavity therein, wherein the cavity extends in depth through the upper conductive oxide coating at least to the lower conductive oxide layer without extending to the substrate, wherein the cavity exposes a portion of the lower conductive oxide layer; and
(E) removing the exposed portion of the lower conductive oxide layer from the bottom of the cavity to expose a region of the substrate, whereby the first and second electrodes, each comprising the lower and upper conductive oxides, and each electrically and laterally separated one from the other by the exposed substrate region, are formed. The first electrode covers a first area of the substrate, and the second electrode covers a second area of the substrate.
In another aspect, the invention provides a first and second electrode prepared by the method set forth above. As mentioned, the first and second electrodes are particularly useful as the source and drain electrodes, respectively, in an OxFET device.
In yet another aspect, the present invention provides a field effect transistor structure, which incorporates the above fabricated electrodes as source and drain electrodes buried beneath active oxides. The structure comprises:
(A) a substrate;
(B) a source electrode comprising a lower conductive oxide disposed atop a first area of the substrate and an upper conductive oxide disposed atop the lower conductive oxide;
(C) a drain electrode laterally separated from the source electrode, wherein the drain electrode comprises the lower conductive oxide disposed atop a second area of the substrate and the upper conductive oxide disposed atop the lower conductive oxide;
(D) a channel oxide layer atop an exposed region of the substrate and atop the source and drain electrodes, wherein the exposed region lies between and laterally separates the source electrode and the drain electrode, wherein the channel oxide layer covering the exposed region forms a channel in the device;
(E) a gate oxide material covering the channel oxide layer;
(F) a first and a second filled contact opening, each extending, respectively, in depth through the gate oxide material and the channel oxide layer to the upper conductive oxide of the source and drain electrodes, and each filled contact opening being filled with a conductive metal; and
(G) a gate electrode disposed atop the gate oxide material, wherein the gate electrode is laterally positioned between the first and second filled contact openings.
In still another aspect, the present invention provides a method for fabricating the field effect transistor device described above. The method comprises:
(A) providing a substrate;
(B) depositing a layer of a lower conductive oxide onto the substrate;
(C) depositing a coating of an upper conductive oxide onto the layer of the lower conductive oxide;
(D) patterning the upper conductive oxide coating to create a cavity therein, wherein the cavity extends in depth through the upper conductive oxide coating at least to the lower conductive oxide layer without extending to the substrate, wherein the cavity exposes a portion of the lower conductive oxide layer;
(E) removing the exposed portion of the lower conductive oxide layer from the bottom of the cavity to expose a region of the substrate, whereby a source electrode and a drain electrode are formed, each comprising the lower and upper conductive oxides, and each electrically and laterally separated one from the other by the exposed substrate region, wherein the source electrode covers a first area of the substrate, and the drain electrode covers a second area of the substrate;
(F) depositing a channel oxide layer onto the exposed substrate region, filling the cavity, and extending onto the upper conductive oxide coating;
(G) depositing a gate oxide material onto the channel oxide layer;
(H) creating a first contact opening and a second contact opening in the gate oxide material, each contact opening extending in depth through the gate oxide material and the channel oxide layer, and terminating, respectively, at the source electrode and the drain electrode;
(I) filling the first and second contact openings with a conductive metal; and
(J) depositing a gate electrode atop the gate oxide material, wherein the gate electrode is positioned laterally between the first and second filled contact openings.
In preferred embodiments, the first and second electrodes are, respectively, the source and drain electrodes of a FET. Furthermore, the substrate preferably comprises strontium titanate (STO), lanthanum aluminate (LAO), strontium lanthanum aluminate (SLAO), neodymium gallinate (NGO), or barium strontium titanate (BST), but most preferably STO. The preferred lower conductive oxide material is lanthanum strontium cobalt oxide, LaSrCoO3 (LSCO) or lanthanum nickel oxide, LaNiO3 (LNO), and the preferred upper conductive oxide material is strontium ruthanate, SrRuO3 (hereinafter, SRO). The preferred method of patterning the upper conductive oxide material is ion milling, and the preferred method of removing the lower conductive oxide material is wet chemical etching by which the upper conductive oxide layer and the substrate are unaffected. Thus, a perovskite oxide channel of good quality can be epitaxially grown on the substrate region exposed by the process.